I would advise using Juvenal's phrase without revision:
Sed quis custodiet ipsos / custodes? (Satire VI, 347-48)
As Lewis & Short remarks in its entry for custos, the term can be used alone to refer to (watch)dogs. Here is one example from Virgil:
Occupat Aeneas aditum custode sepulto,
evaditque celer ripam inremeabilis undae. (Aeneid, VI, 424-25)
Aeneas through hell's portal moved, while sleep
Its warder buried; then he fled that shore
Of Stygian stream, whence travellers ne'er return.
Note that the whole trope is that the verb and noun align, so you would lose the flavor of the passage by reworking it to something like Quis custodiet ipsos canes?
If, however, you specifically want to emphasize that this guard is not a human, then I suppose it is possible to insert some kind of irreverent revision to get this across. (I say "irreverent" fully conscious of the fact that we are modifying a decidedly unscrupulous satire :)
Quis custodiet ipsos canes custodientes?
Note that this revision uses the present participle of custodio: I am not aware of any adjective derived from this word that would work better.